The present invention relates to a disconnect switch circuit, and more particularly to a disconnect switch circuit to power the head retract in a hard disk drive memory.
1. Related Applications
This application is related to the following applications, each of which is being filed on the same day, and each of which is incorporated herein by reference: U.S. application Ser. No. 08,062,503 U.S. application Ser. No. 08,062,968 U.S. application Ser. No. 08,062,969.
2. Description of Prior Art
Moving-media hard disk drive (HDD) memories are commonly used in battery powered portable computers whenever substantial amounts of memory storage are required. In moving-media hard disk drive memories, the motion of a spinning disk suspends a magnetic read/write head above the media to facilitate motion of the head across the disk. A voice coil driver or head actuator is then used to position the head above a particular data sector and track. When the battery fails or is unexpectedly disconnected, emergency power-down procedures including retracting the read/write head to areas free from encoded data must be performed. The positioning of the head into a safe area is known as a "head retract".
If the head retract is not completed before the head crashes onto the disk, permanent loss of data and damage to the disk may result. In order to prevent such a damaging head crash, the head retract circuit and power devices must be powered from an auxiliary power supply. Typically, this auxiliary power supply for performing head retract is created by isolating the flyback electromotive force (emf) generated by the spindle motor in a HDD.
FIG. 1 shows an example of a prior art circuit for preventing crash of the read/write head onto the disk. A Schottky diode 102 is placed in series between a battery 101 and a HDD which includes a spindle circuit 104 and a head actuator circuit 103. Spindle circuit 104, which drives a three-phase spindle motor 130, includes an output stage 120 having three phase-bridges 121, 122, 123, and a spindle control 105. Head actuator circuit 103 includes a head actuator control 141 to control positioning of the head, a head retract circuit 142 to perform the head retract, and an output stage 143 to drive a voice coil motor 144. Under normal operation, battery voltage Vba.sub.bat is applied to the anode of the isolating Schottky diode 102, which is forward-biased and provides a supply voltage Vcc on a voltage supply lead 160. Spindle control 105 converts the D.C. voltage Vcc into a three-phase supply for spindle motor 130. If an unexpected power failure occurs while spindle motor 130 is rotating, Vbat drops to ground and spindle motor 130 becomes a generator due to the momentum of the rotor, generating alternating emf voltage which supply an auxiliary voltage to voltage supply line 160 through the intrinsic antiparallel diodes 131, 133 and 135, acting together as a three-phase rectifier. The auxiliary voltage supply causes Schottky diode 102 to be reverse-biased so as to prevent the residual emf from being drawn by other circuits of the portable computer, thereby isolating the spindle motor 130 from any other circuitry connected to battery 101. As the rotor of motor 130 slows down the emf voltage, and hence the Vcc provided by the auxiliary supply, decrease in amplitude. However, during the time between battery failure and the fall of Vcc, the Vcc created by the emf voltage can be used to perform the head retract and/or other emergency power-down procedures.
A disadvantage of this arrangement is that Schottky diode 102 consumes significant amounts of power under normal operation. It is commonly understood that the forward voltage drop across a Schottky diode is approximately 0.5 volts or higher. It is also known that a typical spindle motor for a hard disk drive draws a current of several amps. Therefore, at least a watt of power is lost due to the Schottky diode 102. In addition, the presence of Schottky diode 102 in the battery path limits the emf of motor 130 to a value below Vbat -0.5V. The back emf voltage in a motor can never exceed the voltage powering it. Therefore a reduction in voltage due to the Schottky drop must reduce the emf voltage on the motor by the same amount. For hard disk drive powered by 5 or even 3 volts, this 0.5 volt drop is detrimental to efficiency and reliable operation. The lower range of Vcc provided by the flyback emf may in some cases prevent an adequate amount of time at useful voltage to facilitate a successful head retract.